1. Field of the Invention
The present invention relates to a discharge plasma processing device.
2. The Prior Art
Plasmas are easily produced by applying a DC and/or AC electric field, and are utilized for many kinds of processing such as cleaning, coating, etching, sputtering and and plasma CVD on the surface of objects to be processed such as substrates and other targets. However, it is difficult to process the surface freely as programmed, for instance, to produce a surface that is sufficiently in uniform, without any other treatment.
A magnetic field is an effective means to localize in space the plasma production. When an electron cyclotron resonance phenomenon is used, plasma is produced at a position of specified magnetic field where the resonance arises, which works as a point plasma source. A planner magnetron discharge which is realized by a special combination of electric field E and magnetic field B near a cathode surface is also useful for sputtering process. However, the eroding efficiency of cathode target used is rather low even when the permanent magnet is set movably.
The very new idea proposed in accordance in with the invention is characterized by the use of plasma produced in a closed or opened magnetic neutral line which is made up with zero magnetic field points connected continuously. The plasma is produced by applying an electric field along the neutral magnetic line for which an alternative AC field is applied in the case of closed loop, while a direct DC field and/or AC field are used for the opened line case.
By choosing an adequate combination of current carrying conductors or a system of permanent magnets for the magnetic field composition, the magnetic neutral line is formed in an arbitrary shape of any size at a variable distance from the object to be processed. As these geometric parameters of the magnetic neutral line can be controlled even during the processing period as well as the plasma parameters can be controlled with rf power input, freely programmed processing, which enables extremely uniform processing, is easily realized.
It is, therefore, an object of the present invention to utilize a plasma produced in a magnetic neutral line for many kinds of plasma processing.
According to an aspect of the invention, the object of the present invention described above is realized by providing a discharge plasma processing device comprising a vacuum chamber with an evacuation system for processing an object with plasma, a magnetic field generation system for generating a magnetic neutral line in the vacuum chamber, and an electric field generation system for applying an electric field along the magnetic neutral line formed in the vacuum chamber in order to produce plasma by discharge along the magnetic neutral line.
For simplicity, a circular configuration is assumed for the following presentation first, since any shape of the closed loop is topologically the same as a circle and the circular configuration has being cited actually for many kinds of plasma processing.
Regarding the formation of a circular magnetic neutral line, the magnetic field generation system preferably may comprise at least two electromagnetic coils of a ring conductor, which are used for the magnetic field generation and are located coaxially but separated keeping a distance. When electric currents flow in steady state in the both coils respectively, a circular magnetic neutral line is formed coaxially with the coils as the result of circularly connected zero magnetic field points. In the case that equal current runs in a same sense in the both coils of a same size, the circular magnetic neutral line appears in the midplane between the two coils, with a smaller radius than that of the coils. When the two coil current values running in the same sense are different from each other, the axial position of the circular magnetic neutral line approaches to the lower current coil, apart from the higher current coil. Even in the case that the current in one coil is in the reversed sense of that in the other coil, the circular magnetic neutral line may appear axially close to the coil of the lower current, apart from the high current coil. Accordingly, the axial position of the circular magnetic neutral line may be regulated by controlling the excitation currents which are fed to the respective magnetic field generation coils.
Regarding the plasma production along the formed circular magnetic neutral line, the electric field generation system preferably may comprise a coil for radio frequency alternating electric field induction with a rf electric power application system, and a rf induction coil may be wound close to but outside to the circular magnetic neutral line. The electric field generating system may alternatively be a single turn coil made of a metal strip or a strip-shaped netting metal wires, having a width large enough to accommodate any axial displacement of the circular magnetic neutral line so as to keep efficient input of electric power for the plasma production in the circular magnetic neutral line.
To make controllable the diameter of the ring plasma produced in the circular neutral magnetic line formed in the midplane between the two coils of a same size in which a same current flows respectively, the third coil of the same size as the first two coils may be preferably disposed coaxially in the midplane. When the current of the third coil flows in steady state in the sense opposite of that of the first two coils, the circular magnetic neutral line splits on the same plane into two concentric circles, inner circle and outer one. The diameter of the inner circle is smaller than that before splitting. As far as plasma is produced in the inner neutral magnetic circle, the diameter of the plasma ring can be controlled by changing the third coil current running in the sense opposite of that of the first two coils.
Then, three circular ring shaped objects exist concentrically in the same midplane: the ring plasma produced in the circular magnetic neutral line innermost, the rf electric field induction coil in the middle, and the third coil for controlling the radius of the circular magnetic neutral line outermost. So that it is necessary to keep effective induction or rf electric field in the circular magnetic neutral line against mutual interference between the currents in both coils due to the geometric adjacency.
To avoid this defect, the difference of current flowing form between steady current of body flow and rf alternating current of surface flow may be preferably applied. A coil having a double layer core structure on the cross-sectional view may be used for that purpose providing a thin electrical insulation zone between the two layers in which the steady current flows inside while the rf alternating current outside.
According to another aspect of the invention, there is provided a discharge plasma processing device for processing an object with plasma, which comprises a vacuum chamber, a hollow cylindrical permanent magnet of thick wall having its N- and S- poles at the both ends respectively, an electromagnetric coil for steady current disposed coaxially and movable along the axis within the hollow cylindrical permanent magnet and a rf current coil to apply an electric field along a circular magnetic neutral line to be formed by the superposition of magnetic field of the hollow cylindrical permanent magnet and that of the steady current coil whose sense is reverse to that of the hollow permanent magnet.
By controlling the current of the steady coil and its axial position, the radius of the circular magnetic neutral line and its axial position can be regulated as programmed.
According to a further aspect of the invention, there is provided a discharge plasma processing device for processing an object with plasma, which comprises a vacuum chamber, a magnetic field generation system for forming a straight magnetic neutral line which is made up with zero magnetic field points connected in turn and an electric field generation system for applying a DC or AC electric field between the both ends of the straight magnetic neutral line in order to produce linear plasma there.
With such an arrangement, the magnetic field generation system may comprise either plural sets of linear current bars or plural sets of linearly arranged permanent magnets.
Regarding the formation of a straight magnetic neutral line, the magnetic field generation system preferably may comprise at least two straight conductor bars for the magnetic field generation, which are set in parallel but separated apart from each other keeping a distance. When electric steady currents flow in a same sense in the both bars respectively, the straight magnetic neutral line is formed at a position between the bars in parallel to them. In the case that an equal steady current flows in the same sense in the both bars, the straight magnetic neutral line appears in the middle between the both bars.
To move the straight magnetic neutral line located at the middle to the direction vertical to the plane determined with the two parallel bars, the third bar of steady current located in parallel to the straight magnetic neutral line in the plane vertical to the two bars plane of the middle point may be preferably used.
When the steady current of the third bar flows in the sense opposite of that of the first two bars, the straight magnetic neutral line moves away from the orignal position with no steady current of the third bar toward the counter direction, apart from the third bar. Accordingly, the vertical position of the straight magnetic neutral line may be regulated by controlling the excitation currents which are applied to the respective magnetic field generation bars.
A long fine plasma column may be produced by applying a DC or AC electric field between two electrodes set at the both ends of the straight magnetic neutral line. The shape of the two electrodes may be an elongated rectangle or ellipse which is long enough to accommodate any vertical displacement of the straight magnetic neutral line to keep efficient input of electric power for the plasma production in the straight magnetic neutral line.
When the DC current excited in the straight magnetic neutral line formed in the middle between the two bars is higher, the long fine plasma column produced results in the long plasma sheet elongated toward the both steady current bars in the case that the DC current flows in the same sense of that in the two bars, while toward the direction perpendicular to the plane where the both bars are located in the case that the DC current flows in the sense opposite to that of the two bars. This sheet shape of current is caused by the DC current itself flowing in the produced plasma. The width of the sheet elongated depends on the ratio of the DC current to that in the two bars. Such a sheet plasma may be also applicable to many of plasma processing like as, coating, sputtering and ashing.
The idea on the use of plasma produced in a closed or opened magnetic neutral line is available not only in the case of circle and straight line but also any other arbitrary shape of any size at a variable distance from the object to be processed. Ellipse, rectangle and S-figure snaked line are also applicable. As an example, a rectangular magnetic neutral loop may be applied for sputtering process with Indium Tin Oxide ITO cathode target of rectangler shape.
In principle, the plasma produced by discharge along a magnetic neutral line is well thermalized and feeds electric power so quickly, because any charged particle accelerated by electric field collides each other so often through its interaction with steeply changing magnetic field in the vicinity of the neutral points of zero magnetic field where any adiabatic condition on particle motion can not be held, in contrary with usual case that suprathermal particles grow by electric field without magnetic field or with parallel magnetic field. Accordingly, such thermalization characteristics of the plasma located in the magnetic neutral line are apt to result in an effective transmission of the electric power applied.
Plasma produced in such a magnetic neutral line diffuses beyond the magnetic field freely but partially prohibited by the magnet field concerning to higher temperature electrons. Electron-neutral collision and ion-neutral collision are dominant in the diffusion.
However, plasmas are being successively generated in the magnetic neutral line as far as electric field is being applied there. The electric power transfer efficiency to plasmas is so high due to the characteristics of quick and well thermalization mechanism as described above.
According to the aimed processing like as cleaning, coating, etching, supttering and plasma CVD, many kinds of processing scheme can be considered by using the plasma produced in the magnetic neutral line.